1. Field of the Invention
This invention relates to apparatus and methods for refractive index modulation, with particular emphasis to the application of refractive index modulation to spatial phase modulation and other transfers of information to an optical beam.
2. Description of the Related Art
Numerous techniques are available which use an optical beam as an information-bearing medium (the term "optical" is intended in its broad sense as indicating both visible light, and electromagnetic radiation on either side of the visible light spectrum.) For example, a change in index of refraction will alter the speed at which light traverses a particular medium, and therefore can be used to introduce a phase shift into the light wavefront. Voltage controlled phase shifting of light is important in many opto-electronic applications. For example, in a spatial light modulator, a voltage-controlled modulation of the index of refraction may be used to cause constructive or destructive interference in a reflected optical beam, depending upon the voltage-controlled phase shift.
A common form of spatial light modulator is a liquid crystal light valve. The development and theory underlying light valve technology is illustrated in patents such as U.S. Pat. No. 3,824,002, issued to T. D. Beard on July 16, 1974, and U.S. Pat. No. 4,019,807, issued to D. D. Boswell et al. on Apr. 26, 1977. These patents are assigned to Hughes Aircraft Company, the assignee of the present invention. However, the liguid crystals used in these devices have response times on the order of milliseconds, making them unsuitable for higher speed applications in conjunction with high speed materials like GaAs. A higher speed modulation device would be very desirable for adaptive optics, optical processing applications and integrated optics.
Guided wave switching and modulation in integrated optics is another area in which a material with a variable refractive index can be used to apply information to an optical beam. Optical phase shifting for guided wave applications is done mainly with LiNbO.sub.3 as the optical medium, but it is not easily integrated with high speed GaAs.
One type of structure that has been developed and proposed for various optical applications is a "nipi" doping superlattice. This type of device was originally proposed as a lamination of n- and p-doped semiconductor layers with intrinsic zones in between the layers (the layer sequence n-intrinsic-p-intrinsic has been popularly abbreviated to "nipi".) Studies have indicated that nipi structures can exhibit exotic properties that are shown by neither bulk crystals nor compositional superlattices. Although the experimental work and later theoretical studies have generally dealt with doping structures that do not contain intrinsic regions, the term "nipi" is generally understood as referring to the entire class of doping superlattices, with or without intrinsic regions.
The unusual electronic properties of nipi structures derive from the different nature of the superlattice potential, specifically the space charge potential of ionized impurities in the doping layers. This is in contrast to compositional superlattices, composed of laternating layers of different materials or compositions, in which the superlattice potential originates from the different bandgap values of their components. The space charge potential in the nipi doping superlattice modulates the band edges of the host material such that electrons and holes become spatially separated. This separation can be optimized by appropriate doping concentrations and layer thicknesses. As a result of the strong spatial separation, excess-carrier recombination lifetimes can be larger by many orders of magnitude than those in the host material. Large excess carrier concentrations can be achieved easily, either by relatively weak optical excitation or by low injection currents. The spatial separation between ionized donors and acceptors results in an alternating space charged potential. With increasing carrier concentration both the number of ionized donors and acceptors and the amplitude of the superlattice potential decreases while, at the same time, the effective bandgap increases. Associated with this tunability of the bandgap is a tunability of the recombination lifetime, owing to a lowering of the tunneling or thermal barriers for recombination.
The tunability of carrier concentration, bandgap and lifetimes may be inferred to give rise to a tunability of electron and hole conductivity, of the spectra for luminescence, stimulated emission and absorption, and of the refractive index.
A basic discussion of the nipi structure is given in an article by Klaus Ploog and Gottfried H. Dohler, "Compositional and Doping Superlattices in III-V Semiconductors", Advances In Physics, Vol. 32, No. 3, 1983, pages 285-359. This article presents a general discussion of nipi's, as well as the spatial control of optical absorption by a voltage pattern applied to the nipi. Other applications, such as in photoconductors, photodiodes, ultrafast photodetectors, light emitting devices and optical absorption modulators, are discussed in an article by Dohler, "The Potential of n-i-p-i Doping Superlattices For Novel Semiconductor Devices", Superlattices and Microstructures, Vol. 1, No. 3, 1985, pages 279-287. A further expansion on nipi applications is provided in another article by Dohler, "Light Generation, Modulation, and Amplification by n-i-p-i Doping Superlattices", Optical Engineering, Vol. 25, No. 2, February, 1986, pages 211-218.
While various applications for nipi structures have been postulated, there is not believed to be any prior art linkage between nipi's and spatial phase modulation, or guided wave switching and modulation. A faster, reliable technique for accomplishing these functions is still needed.